Hormone and neurotransmitter release are activated by a transient rise in cytosolic free calcium (Ca). The cellular mechanisms regulating the rise, magnitude and decay of the Ca signal consequently influence the magnitude and duration of the secretory response. While the mechanisms mediating the rise of intracellular Ca are understood, the mechanisms regulating the decay of the Ca signal are unknown. Recent evidence suggests that sodium-calcium exchanger (NCX) proteins significantly contribute to the decay of the of the Ca signal for neurosecretory response. Specifically, previous studies suggest a novel mechanism influencing the decay of the local Ca signal for neurosecretion which uses the insertion of secretory vesicle NCX proteins into the plasma membrane during exocytotic secretion. The aim of this study is to identify and compare of the NCX proteins present on the secretory vesicle membrane and plasma membrane in the prototypic neurosecretory cell, the adrenal chromaffin cell. The NCX proteins will be characterized utilizing molecular biology, Western blot and immunocytochemical methods and transport studies. The long term goal of this project is to determine whether a NCX isoform with distinct transport properties is localized to secretory vesicles and contributes to Ca signal decay. The examination of this novel model is of particular interest because it may also be active in nerve terminals and other secretory cells. The properties of the chromaffin cell NCX protein are relevant to clinical situations involving altered ion levels and the sympathoadrenal stress response, such as essential hypertension and cardiac glycoside administration.